The invention relates generally to flat panel displays. More particularly, the present invention relates to a method and apparatus for minimizing diffraction effects in flat panel displays.
Flat panel displays enjoy wide appeal as computer screens, television screens, electronic game displays, avionics or vehicular displays, and as displays in a variety of other applications because of their light weight, small footprint, relatively sharp resolution, and low power consumption. Unfortunately, flat panel displays can exhibit undesirable visual effects previously unseen with other types of display devices. One such visual effect is the presence of bright bands, or multiple images, surrounding the mirror image of a small, non-diffused, intense light source, such as the sun.
The visual anomalies, as shown on a flat panel display 10 in FIG. 1, generally lack well-defined features and instead, appear as broad, orthogonal bands 24 extending vertically and horizontally away from the mirror or specular image 20 of the small, intense light source 18 and may also include multiple secondary images 22 of the light source 18. Bands 24 and the secondary images 22, collectively referred to as a diffraction pattern, always surround the specular image 20 of the light source 18, with the diffraction pattern being brightest near the specular image 20.
For example, in the case of flat panel display 10 being a liquid crystal display (LCD) 240 (see FIG. 2), it is the sharp boundary between a black matrix 244 and a color filter 246 that is the primary source of the diffraction pattern. An incident ray 80 impinging on a region of LCD 240 where no black matrix 244 is present experiences Fresnel reflectance and transmittance to generate a reflection ray 82 and a transmission ray 84, respectively. At regions of black matrix 244 away from its edges, an incident ray 86 will experience Fresnel reflectance to generate a reflection ray 88, but no transmission ray because black matrix 244 is optically opaque. Thus all points on LCD 240 not on an edge of black matrix 244 lead to Fresnel reflectance that is specular because only a small refractive index difference exists between a front substrate 242 and the color filter 246.
In contrast, incident rays 90, 100 impinging on an edge of black matrix 244 leads to Fresnel reflectance and transmittance to generate respectively reflection rays 92, 102 and transmission rays 94, 104 as described above, but also reflection-mode diffraction rays 96, 106 and transmission-mode diffraction rays 98, 108. It is at these edges that the difference in the Fresnel reflectance at the boundary of the black matrix 244 and color filter 246 leads to the diffraction pattern. Specifically, it is the diffraction rays exiting the front of LCD 240, e.g., reflection-mode diffraction rays 96, 106, that are problematic since LCD 240 is viewed from a viewing direction 258.
Typically, the appearance of the diffraction pattern, i.e., intensity, shape, color, etc., is directly correlated to the physical construction of the flat panel display, and not its operation. For example, displays of the same design exhibit similar diffraction patterns, while displays of a different design may exhibit a significantly different diffraction pattern. Similarly, one display design may generate predominantly chromatic diffraction patterns, while other display designs may lead to strong spectral dispersion with repeating bands. Furthermore, the brightness of the diffraction pattern may vary according to the display design.
In addition to the sun, other small, intense light sources (also referred to as point sources) such as incandescent or arc lamps can also generate diffraction patterns on flat panel displays. Basically, light sources with limited angular size are potential diffraction pattern generators, while spatially extended sources like fluorescent or diffused lamps or sunlight reflecting from diffuse surfaces such as clothing or clouds do not generate diffraction patterns as shown in FIG. 1. This is not to say that spatially extended sources, also referred to as area sources, do not produce diffraction patterns. Instead, each point on the area source generates its own diffraction pattern with a slight lateral displacement thereof. Then, when the diffraction patterns from all the points visually superimpose, the peaks in each of the diffraction patterns merge to form secondary images of the area source. Hence, area sources do not produce significant diffraction effects as observed with point sources such as the sun.
Moreover, it has been observed that changing the orientation of the flat panel display and/or viewing direction does not eliminate the diffraction pattern. For example, when the flat panel display is rotated relative to the viewer and the light source, the diffraction pattern also rotates with the display. Similarly, when the viewer moves relative to the display and light source, the diffraction pattern will also shift in position on the display. Hence, if the diffraction pattern is bright enough over a relevant portion of the display, the readability of the information presented by that portion of the display will be impaired. Display readability degradation results in increased reading time, decreased reading accuracy, and viewer discomfort due to eyestrain, and even possibly headache and nausea. Diffraction patterns are particularly problematic in display applications where the positions of the display, viewer, and light source are not independently controllable such as in avionics or vehicle applications. In such situations, for example, in modern fighter jet cockpits with large transparent canopies, the operator (i.e., the pilot or driver) may experience performance degradations due to increased reading time or decreased reading accuracy due to the diffraction pattern.
Thus, there is a need for an apparatus and method for minimizing diffraction effects on flat panel displays. Moreover, given the complex structure of flat panel displays, there is a need for the apparatus and method to minimize such diffraction effects without adding undue weight to the displays or requiring extensive changes to their physical construction thereby negating desirable performance parameters of existing displays. Still further, there is a need for the apparatus and method to be easily modifiable for implementation in various types of flat panel displays.
One embodiment of the invention relates to a flat panel display system having a front viewing surface. The system includes a display layer, a first electrode, and a second electrode. At least one of the display layer, the first electrode, and the second electrode includes an edge having an edge texture, wherein the edge texture minimizes diffraction effects visible on the front viewing surface of the system caused by a point light source.
Another embodiment of the invention relates to a flat panel display system having a front viewing surface. The system includes a black matrix, a first electrode, a second electrode, a color filter, and an electrical structure. At least one of the black matrix, the first electrode, the second electrode, the color filter, and the electrical structure includes an edge having an edge texture, wherein the edge texture minimizes diffraction effects visible on the front viewing surface of the system caused by a point light source.
Another embodiment of the invention relates to a flat panel display including a plurality of display elements, each display element having a first structural feature with an edge and a second structural feature disposed on the first structural feature, wherein incident rays from a point light source incident on the edge of the first structural feature and then the boundary of the second structural feature generates undesirable diffraction effects. The display further includes means for minimizing the undesirable diffraction effects. In one preferred embodiment, the means for minimizing the undesirable diffraction effects includes an edge texture included on the edge of the first structural feature. In still another preferred embodiment, the edge texture includes a profile of the edge texture selected from a group including a fractal profile, a discontinuous curved profile, a continuous curved profile, a random profile, a piecewise linear profile, a non-linear profile, and a profile configured to decrease the diffraction effects caused by a smooth surface of the edge.
Still another embodiment of the invention relates to a method for minimizing diffraction effects on a flat panel display. The method includes generating an edge texture on an edge of a structural feature of a display element of the display, wherein the edge minimizes diffraction effects when exposed to a point light source.